1. Field of the Technology
The present invention relates to wireless communication technologies, and more particularly, to methods and apparatuses for space-time coding and decoding.
2. Background of the Technology
Along with the development of wireless communication technologies, a wireless communication system is expected to support higher communication quality and a higher data rate. In the wireless communication system, time and frequency resources are limited, so that a Multiple Input Multiple Output (MIMO) system is developed. And extensive attention is paid to the MIMO system which provides remarkable improvement in channel capacity.
Diversity techniques are generally adopted in MIMO systems for achieving better signal transmission quality. However, since size of a mobile terminal is limited and there is not enough space for multiple antennas to ensure independence of communication channels, receiver diversity can not be implemented in downlink channels. Base stations are capable of processing multiple transmission antennas, so that space-time coding technologies are usually adopted for implementing transmission diversity in the wireless communication system with multiple antennas.
At present, a Space-Time Block Code (STBC) has become a main technology adopted for implementing transmission diversity, because the STBC has relatively low complexity to implement.
However, bit error rates (BERs) of systems remain high when conventional STBC algorithms are adopted. Therefore, receiver performance of the systems is poor and development of the wireless communication systems with multiple antennas is limited.
Further, when signals to be transmitted in a system are at least two complex numbers, the transmission rate of the system is relatively low.
Taking the STBC for two antennas as an example, a transmission matrix is:
      (                                        x            1                                                x            2                                                            -                          x              2              *                                                            x            1            *                                )     
Here, x1 and x2 respectively indicate two signals to be transmitted in a base station, and * indicates a conjugate operation.
FIG. 1 shows a structure of an STBC algorithm. Detailed descriptions of the algorithm are shown as follows.
In Time Period 1, x1 is transmitted to Mobile Terminal via Antenna 1 and x2 is transmitted to Mobile Terminal via Antenna 2 in Base Station, and signals received by the Mobile Terminal are:r1=h1x1+h2x2+n1 
In Time Period 2, −x*2 is transmitted to Mobile Terminal via Antenna 1 and x*1 is transmitted to Mobile Terminal via Antenna 2 in Base Station, and signals received by Mobile Terminal are:r2=−h1x*2+h2x*1+n2 
Here, h1 indicates a parameter of a flat fading channel from Antenna 1 to Mobile Terminal and h2 indicates a parameter of a flat fading channel from Antenna 2 to Mobile Terminal, i.e. channel status information, rm and nm respectively indicate the signals and Additive White Gaussian Noise (AWGN) received by Mobile Terminal in Time Period m (m=1,2).
The receiver computes signals to be tested, i.e. {circumflex over (x)}1 and {circumflex over (x)}2, as follows:{circumflex over (x)}1=h*1r1+h2r*2 {circumflex over (x)}2=h*2r1−h1r*2 
Finally, Mobile Terminal may respectively perform detection for the original signals sent by Base Station, i.e., x1 and x2, according to the maximum likelihood rule.
The STBC algorithm is further explained with reference to a system with four antennas. A transmission matrix is:
      [                                        x            1                                                x            2                                                x            3                                                x            4                                                            -                          x              2                                                            x            1                                                -                          x              4                                                            x            3                                                            -                          x              3                                                            x            4                                                x            1                                                -                          x              2                                                                        -                          x              4                                                            -                          x              3                                                            x            2                                                x            1                                                            x            1            *                                                x            2            *                                                x            3            *                                                x            4            *                                                            -                          x              2              *                                                            x            1            *                                                -                          x              4              *                                                            x            3            *                                                            -                          x              3              *                                                            x            4            *                                                x            1            *                                                -                          x              2              *                                                                        -                          x              4              *                                                            -                          x              3              *                                                            x            2            *                                                x            1            *                                ]    ,
Here, x1, x2, x3 and x4 respectively indicate four signal stobe transmitted in a base station, and * indicates a conjugate operation.
Signals received by a mobile terminal in 8 consecutive time periods are as follows:r1=h1x1+h2x2+h3x3+h4x4+n1 r2=−h1x2+h2x1−h3x4+h4x3+n2 r3=−h1x3+h2x4+h3x1−h4x2+n3 r4=−h1x4−h2x+h3x2+h4x+n4 r5=h1x*1+h2x*2+h3x*3+h4x*4+n5 r6=−h1x*2+h2x*1−h3x*4+h4x*3+n6 r7=−h1x*3+h2x*4+h3x*1−h4x*2+n7 r8=−h1x*4−h2x*3+h3x*2+h4x*1+n8 
Here, hm indicates a parameter of a flat fading channel from a antenna m (m=1, 2, 3, 4) in a base station to the mobile terminal, and rm and nm respectively indicate the signals and Additive White Gaussian Noise received by the mobile terminal in the time period m(m=1, 2, 3, 4, 5, 6, 7, 8).
The system with four antennas computes the four signals to be tested, i.e. {circumflex over (x)}1, {circumflex over (x)}2, {circumflex over (x)}3, {circumflex over (x)}4, as follows:{circumflex over (x)}1=h*1r1+h*2r2+h*3r3+h*4r4+h1r*5+h2r*6+h3r*7+h4r*8 {circumflex over (x)}2=h*2r1−h*1r2−h*4r3+h*3r4+h2r*5−h1r*6−h4r*7+h3r*8 {circumflex over (x)}3=h*3r1+h*4r2−h*1r3−h*2r4+h3r*5−h4r*6−h1r*7−h2r*8 {circumflex over (x)}4=h*4r1−h*3r2+h*2r3−h*1r4−h4r*5−h3r*6−h2r*7−h1r*8 
It can be seen that, when signals to be transmitted are complex numbers and the number of antennas is larger than two in an MIMO system, the transmission rate of the system is half of the transmission rate of a Single Input Single Output (SISO) system, thus the transmission rate of the MIMO system is reduced.
Therefore, when the conventional STBC algorithm is adopted in the system, the bit error rate of the system is high. Further, the transmission rate of the system is low when signals to be transmitted are more than one complex numbers.